Bananas under attack: Understanding their foes

New genetic studies could help researchers gain the upper hand against fungi that infect bananas

Farmers are growing bananas battling diseases that threaten the popular yellow-skinned fruit. Fungi cause some killer plant blights. And new analyses show these pathogens are growing nastier. Some of the genes in the fungi are changing in ways that allow those fungi to steal the banana’s nutrients. The new data could help researchers breed bananas to thwart the fungal threats.

Rest assured, you won’t get sick eating bananas that came from infected plants. The fungi spread by air and only attack the plants’ leaves. The infected greenery develops small yellow spots. Over time, they darken and expand. The infected tissue eventually dies. Less tissue that’s healthy will slow the plants’ photosynthesis. This is the process by which plants use sunlight to make food. So infected plants produce fewer bananas.

Farmers fight fungal diseases with chemical sprays. But those sprays are expensive. In fact, they can add up to nearly a third of the cost of growing bananas. Those chemicals also can harm the environment. Even worse, the fungi have begun to resist the killing effects of certain sprays. So researchers are looking for new ways to control the fungal infections.

One of the culprits behind the banana blight is a disease known as Black Sigatoka (SEEG-uh-TOKE-uh). It takes its name from a town on the South Pacific island nation of Fiji. That’s where the blight first turned up in the 1960s. Since then, this fungus has spread widely around the world. So have two related fungal species. They cause Yellow Sigatoka and Eumusae (Yu-MYOO-say) leaf spot.

This banana leaf is infected with Black Sigatoka fungus.

Neil Palmer/Wikimedia Commons (CC BY-SA 2.0)

All three diseases look similar, but Black Sigatoka poses the biggest threat. It afflicts a wide range of bananas, including the “Cavendish” variety. This is the sweet “dessert” banana enjoyed in most Western countries. Yellow Sigatoka is a milder disease. Eumusae leaf spot falls somewhere in between the two Sigatokas in terms of threat.

The fungi are closely related. So researchers at the University of California, Davis were puzzled about why they cause such different degrees of harm.

To find out, they worked with researchers at Wageningen University in the Netherlands. The team mapped out the complete set of genes, or genome, of all three fungi. Like other organisms, fungi will, over time, develop mutations — changes in their DNA. Some genetic tweaks could help the fungi survive. Such beneficial DNA changes would likely carry over to future generations.

So to learn how the fungi had adapted to their banana hosts, the researchers looked at which genes in them had changed. They also noted which mutations occurred in the two more damaging (or virulent) fungi.

The shared changes showed up in genes that control the chemical reactions that occur within a banana plant’s cells. Those life-sustaining reactions are known as an organism’smetabolism. The genes help Black Sigatoka and Eumusae fungi break down cell walls in banana leaves. That allows the fungi to steal nourishment from infected cells.

The new analysis suggests these fungi “evolved a more aggressive way to get nutrients from the host,” says Ti-Cheng Chang. He is a computational biologist at St. Jude Children’s Research Hospital in Memphis, Tenn.

Chang and his coworkers published their findings August 11 in PLOS Genetics.

The findings offer food for thought. Could scientists disable some of the banana’s genes to keep fungi from thwarting those chemical reactions? Doing so might slow the infection's spread.

“It’s a question a number of us are asking,” says James Dale. He’s a biotechnologist at Queensland University of Technology in Australia. He has tinkered with banana genes to create disease-resistant varieties of the fruit.

The new study also looked at other genes that direct the fungi to make special proteins. Those proteins appear to help the pathogen infect banana leaves.

Despite causing similar banana diseases, the three killer fungi have evolved unique sets of virulence proteins. That suggests they each developed different ways to outsmart theimmune system of a banana plant. Another study in the same journal has more details on those proteins in Black Sigatoka.

All told, the new data pinpoint specific molecules at play in the infections that threaten some types of popular bananas. “What sorts of things could we do to manipulate the plant so it becomes resistant [to them]?” asks Dale. That, he says, is “clearly where we need to go.”

Power Words

biotechnologist A scientist who uses living cells to make useful things.

blight A devastating disease that affects the growth or survival of plants. The term is sometimes extended to environmental conditions (such as locusts or drought) that can also imperil crops, forests or other valued plants.

breed (noun) Animals within the same species that are so genetically similar that they produce reliable and characteristic traits. German shepherds and dachshunds, for instance, are examples of dog breeds. (verb) To produce offspring through reproduction.

cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

computational biology A field in which scientistst use mathematics and computer programs to better understand living things.

data Facts and/or statistics collected together for analysis but not necessarily organized in a way that give them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

environment The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.

fruit A seed-containing reproductive organ in a plant.

fungus (plural: fungi) One of a group of single- or multiple-celled organisms that reproduce via spores and feed on living or decaying organic matter. Examples include mold, yeasts and mushrooms.

gene (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

generation A group of individuals born about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet are referred to as belonging to a particular generation of humans. The term is sometimes also extended to inanimate objects, such as electronics or automobiles.

genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

genome The complete set of genes or genetic material in a cell or an organism. The study of this genetic inheritance housed within cells is known as genomics.

host (in biology and medicine) The organism in which another lives. Humans may be a temporary host for food-poisoning germs or other infective agents.

immune system The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.

infect To spread a disease from one organism to another. This usually involves introducing some sort of disease-causing germ to an individual.

infection A disease that can spread from one organism to another. It’s usually caused by some sort of germ.

metabolism The set of life-sustaining chemical reactions that take place inside cells and bigger structures, such as organs. These reactions enable organisms to grow, reproduce, move and otherwise respond to their environments.

molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

mutation (v. mutate) Some change that occurs to a gene in an organism’s DNA. Some mutations occur naturally. Others can be triggered by outside factors, such as pollution, radiation, medicines or something in the diet. A gene with this change is referred to as a mutant.

nutrient A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.

pathogen An organism that causes disease.

photosynthesis (verb: photosynthesize) The process by which green plants and some other organisms use sunlight to produce foods from carbon dioxide and water.

proteins Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better-known, stand-alone proteins. Medicines frequently work by latching onto proteins.

range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists. (in math or for measurements) The extent to which variation in values is possible. Also, the distance within which something can be reached or perceived.

species A group of similar organisms capable of producing offspring that can survive and reproduce.

tissue Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.

variety (in agriculture) The term that plant scientists give to a distinct breed (subspecies) of plant with desirable traits. If the plants were bred intentionally, they are referred to as cultivated varieties, or cultivars.

virulence (in medicine) The potency of a virus, bacterium or other agent in causing infectious disease. Among a given species, some strains may cause disease with very little exposure (such as infection with a few cells). Less virulent strains may take massive exposures to create disease.